Audit short-term: currency preview, date bounds, success destination, safe-area

- H3: Live currency preview below currency inputs shows the value formatted
  with thousands separators (en-US, USD) using Intl.NumberFormat. Skipped
  inside matrix cells to keep the Y1 monthly table compact.
- M1: Date inputs now apply min/max bounds. Default window is 1900-01-01 to
  2100-12-31; per-field override via FieldConfig.min/max as ISO strings.
- H6: On successful submit, replace the form with a SuccessDestination card
  (large checkmark, org name, "Submit another" + "safe to close" affordance).
  Prevents accidental duplicate submits from back-button / autofill replay.
- M6: Sticky submit bar respects iOS safe-area-inset-bottom.

FieldRenderer now takes a control prop so the currency preview can subscribe
to its single field via useWatch without re-rendering the whole form.

components/StageSection.tsx

@@ -2,7 +2,12 @@
 
 import { useEffect, useId, useMemo, useState } from "react";
 import type { StageSectionConfig, SelectOption } from "@/types/form";
-import type { UseFormRegister, FieldValues, FieldErrors } from "react-hook-form";
+import type {
+  UseFormRegister,
+  FieldValues,
+  FieldErrors,
+  Control,
+} from "react-hook-form";
 import { FieldRenderer } from "./fields/FieldRenderer";
 import { MatrixGroup } from "./MatrixGroup";
 import { StageIcon } from "./StageIcon";
@@ -11,6 +16,7 @@ import { evaluate } from "@/lib/conditional";
 interface StageSectionProps {
   section: StageSectionConfig;
   register: UseFormRegister<FieldValues>;
+  control: Control<FieldValues>;
   errors: FieldErrors;
   /** Live form values, used to evaluate per-field visibility rules. */
   formValues: Record<string, unknown>;
@@ -36,6 +42,7 @@ interface StageSectionProps {
 export function StageSection({
   section,
   register,
+  control,
   errors,
   formValues,
   isCurrent,
@@ -153,6 +160,7 @@ export function StageSection({
                     <FieldRenderer
                       field={f}
                       register={register}
+                      control={control}
                       errors={errors}
                       readonlyValue={formValues[f.name]}
                       resolvedOptions={resolvedOptions}